Plugin management is server-admin scoped, and on an SSO hive nobody holds that
role: auto_assign_org_role grants an org role, and the built-in local admin
that does hold server admin cannot log in because the login form is disabled
whenever SSO is configured. Two individually-correct decisions leaving no path
to the plugin UI at all.
Declarative is the way through rather than a workaround for it -- plugins land
in the store and in git, survive a rebuild and a state reset, and the container
needs no runtime egress to grafana.com.
mkIf rather than passing the list through: upstream's default is null, while an
empty list is a real value pointing the plugin path at an empty store dir, so a
hive that sets nothing must keep seeing null.
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